Optical information recording medium and method for producing the same

ABSTRACT

An optical information recording medium includes a first substrate; at least a first dielectric layer and a recording layer for signal recording provided on a surface of the first substrate; and a second substrate. The first substrate and the second substrate are assembled together in the state of being warped in planar symmetry and flattened. The first dielectric layer and the recording layer is interposed between the first substrate and the second substrate.

This is a division of copending application Ser. No. 09/120,081, filedJul. 21, 1998 now U.S. Pat. No. 6,165,578 issued Dec. 26, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a rewritable optical informationrecording medium including two substrates assembled together and amethod for producing the same.

2. Description of the Related Art

Technologies for performing high-density recording of information andreproduction of such information using laser beams are known. Forexample, optical disks have been put into practical use. Optical disksare roughly classified into read-only type, write-once type, andrewritable type. The read-only type is realized as products such as, forexample, compact disks having musical information stored therein andlaser disks having image information stored therein. The write-once typeis realized as products such as, for example, document files and stillpicture files. Currently, research and development is mainly performedon rewritable type. The rewritable type is being realized as productssuch as, for example, data files for personal computers.

A general optical disk has a transparent resin substrate having athickness of 1.2 mm, a recording layer provided on one surface of thesubstrate, and a protective layer such as, for example, an overcoatprovided on the recording layer. Another general optical disk includes asubstrate and a protective plate formed of the same material as thesubstrate which are assembled together with an adhesive.

In order to increase the recording density of optical disks, studieshave been recently performed for shortening the wavelength of the laserlight and using an objective lens having a large numerical aperture(NA). However, as the wavelength of the laser light is shortened and thenumerical aperture is increased, the tolerance for the angle of the diskwith respect to the incident angle of the laser light (referred to asthe “tilt”) is reduced. Reducing the substrate thickness (distance fromthe surface of the substrate to the recording layer) is effective inincreasing the tolerance for the tilt. For example, the substratethickness of a digital video disk (DVD) is 0.6 mm. Since the resinsubstrate having a thickness of 0.6 mm is insufficient in mechanicalstrength, two such substrates are assembled with the recording layerinterposed therebetween.

The substrates are assembled by various methods, such as, for example,applying a hot melt resin on a surface of one substrate and then puttingthe substrates into contact with each other, assembling the twosubstrates with an adhesive tape (two-sided tape) interposedtherebetween, or applying a ultraviolet (UV) curable resin on a surfaceof one substrate, then putting the two substrates into contact with eachother and curing the resin with ultraviolet rays.

It has been found that a substrate having a thickness of as small as 0.6mm is significantly warped when provided with a thin layer including arewritable recording layer on a surface of the substrate.

Such a phenomenon does not occur when a substrate having a thickness of0.6 mm is provided with a thin layer including a read-only recordinglayer, i.e., a metal reflective layer (formed of, for example, Al or Au)on a surface of the substrate. The phenomenon does not occur with asubstrate having a thickness of 1.2 mm regardless of a thin film,including a rewritable recording layer or a read-only recording layer isprovided.

The cause of the warp is considered to be as follows. In the case of therewritable type disks, a dielectric layer is included in the thin layerin addition to the recording layer for protecting the recording layer.Formation of the dielectric layer generates a large stress, and thestress warps the substrate which has an insufficient mechanicalstrength.

When such a significantly warped substrate and another substrate whichis not warped are assembled together to product an optical disk capableof one-side recording and reproduction, the resultant optical disk isalso warped. Such a warped optical disk is not usable.

When two substrates which have the same thin layers and thus are warpedat the same degree are assembled, a highly flat optical disk is obtainedbecause the stresses generated in both substrates are balanced. However,use of two substrates both having the recording layers for producing anoptical disk for one-side recording disadvantageously raises theproduction costs.

SUMMARY OF THE INVENTION

According to one aspect of the invention, an optical informationrecording medium includes a first substrate; at least a first dielectriclayer and a recording layer for signal recording provided on a surfaceof the first substrate; and a second substrate. The first substrate andthe second substrate are assembled together in the state of being warpedin planar symmetry with respect to each other and flattened, the firstdielectric layer and the recording layer being interposed between thefirst substrate and the second substrate.

In one embodiment of the invention, an optical information recordingmedium further includes a thin layer provided on a surface of the secondsubstrate, the surface being opposed to the first substrate.

In one embodiment of the invention, the thin layer includes a seconddielectric layer.

In one embodiment of the invention, the thin layer includes a seconddielectric layer formed of an identical material as that of the firstdielectric layer provided on the first substrate.

In one embodiment of the invention, the thin layer includes a metallayer.

In one embodiment of the invention, the thin layer has such a thicknessto warp the second substrate to substantially an equal degree with thefirst substrate.

In one embodiment of the invention, an optical information recordingmedium further includes a resin layer formed of a resin provided on asurface of the second substrate which shrinks when being cured, theresin layer being provided on the surface not facing the firstsubstrate.

In one embodiment of the invention, the resin layer is formed of a UVcurable resin.

In one embodiment of the invention, the resin layer is transparent withrespect to UV light.

In one embodiment of the invention, the resin layer is non-transparentwith respect to visible light.

In one embodiment of the invention, the resin layer has graphicsthereon.

In one embodiment of the invention, the second substrate is warped whilebeing formed.

In one embodiment of the invention, the second substrate is formed by aninjection method.

In one embodiment of the invention, at least one of the first substrateand the second substrate has a thickness of about 0.8 mm or less.

According to one aspect of the invention, an optical informationrecording medium includes a first substrate; at least a first dielectriclayer and a recording layer for signal recording provided on a surfaceof the first substrate; a second substrate; and a thin layer on which asignal is unrecordable, the thin layer being provided on a surface ofthe second substrate. The first substrate and the second substrate areassembled together with the first dielectric layer, the recording layerand the thin layer being interposed therebetween.

In one embodiment of the invention, the thin layer includes a seconddielectric layer.

In one embodiment of the invention, the thin layer includes a seconddielectric layer formed of an identical material as that of the firstdielectric layer provided on the first substrate.

In one embodiment of the invention, the first dielectric layer and thesecond dielectric have a substantially equal thickness.

According to one aspect of the invention, an optical informationrecording medium includes a first substrate; a plurality of firstdielectric layers, a recording layer for signal recording providedbetween the plurality of dielectric layers, and a metal layer, which areprovided on a surface of the first substrate; a second substrate; and athin layer, including at least one second dielectric layer and a metallayer, on which a signal is unrecordable, the thin layer being providedon a surface of the second substrate. The first substrate and the secondsubstrate are assembled together with the plurality of first dielectriclayers, the at least one second dielectric layer, the recording layer,the metal layer, and the thin metal layer being interposed therebetween.

In one embodiment of the invention, the plurality of first dielectriclayers and the at least one second dielectric layer has a substantiallyequal total thickness.

According to one aspect of the invention, an optical informationrecording medium includes a first substrate; at least a dielectric layerand a recording layer for signal recording provided on a surface of thefirst substrate; a second substrate; and a resin layer formed of a resinwhich shrinks when being cured, the resin layer being provided on asurface of the second substrate. The first substrate and the secondsubstrate are assembled together with the dielectric layer and therecording layer being interposed therebetween, and the resin layer isnot interposed therebetween.

In one embodiment of the invention, the resin layer is formed of a UVcurable resin.

According to one aspect of the invention, an optical informationrecording medium includes a first substrate; at least a dielectriclayer, a recording layer for signal recording provided on a surface ofthe first substrate, and a first resin layer formed of a resin whichshrinks when being cured; and a second substrate. The first substrateand the second substrate are assembled together with the dielectriclayer, the recording layer and the first resin layer being interposedtherebetween.

In one embodiment of the invention, an optical information recordingmedium further includes a second resin layer having a smaller tensilestrength than that of the first resin layer, the second resin layerbeing provided on a surface of the second substrate, the second resinlayer being provided on the surface facing the first substrate.

In one embodiment of the invention, the second resin layer has a smallerthickness than that of the first resin layer.

In one embodiment of the invention, the second resin layer istransparent with respect to UV light.

In one embodiment of the invention, the second resin layer isnon-transparent with respect to visible light.

In one embodiment of the invention, the second resin layer has graphicsthereon.

In one embodiment of the invention, at least one of the first substrateand the second substrate has a thickness of about 0.8 mm or less.

According to one aspect of the invention, a method for producing anoptical information recording medium includes the steps of forming atleast a first dielectric layer and a recording layer for signalrecording on a surface of a first substrate; warping a second substrate;and assembling the first substrate, which is warped, and the secondsubstrate in planar symmetry with respect to each other, and flatteningan assembly of the first substrate and the second substrate.

In one embodiment of the invention, the step of warping the secondsubstrate includes the step of forming a thin layer on a surface of thesecond substrate, the surface being opposed to the first substrate.

In one embodiment of the invention, the step of forming the thin layerincludes the step of forming a second dielectric layer.

In one embodiment of the invention, the step of forming the thin layerincludes the step of forming a second dielectric layer formed of anidentical material with that of the first dielectric layer.

In one embodiment of the invention, the step of forming the thin layerincludes the step of forming a metal layer.

In one embodiment of the invention, the step of warping the secondsubstrate includes the step of forming a resin layer of a resin whichshrinks when being cured on a surface of the second substrate, the resinlayer being formed on the surface not facing the first substrate.

In one embodiment of the invention, the step of forming the resin layerincludes the step of forming a UV curable resin layer.

In one embodiment of the invention, the step of warping the secondsubstrate includes the step of causing a warp to the second substratewhile the second substrate is being formed by an injection method.

In one embodiment of the invention, the second substrate is formed by aninjection method.

According to one aspect of the invention, a method for producing anoptical information recording medium includes the steps of forming atleast a dielectric layer and a recording layer for signal recording on asurface of a first substrate; forming a resin layer of a resin whichshrinks when being cured on the recording layer; and assembling thefirst substrate and a second substrate opposed to the first substratewith the dielectric layer and the recording layer being interposedtherebetween.

Thus, the invention described herein makes possible the advantages ofproviding a flat and low-cost optical information recording medium, forone-side recording and reproduction, including two thin substratesassembled together, and a method for producing the same.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1F are cross-sectional views illustrating a method forproducing an optical disk according to a first example of the presentinvention;

FIG. 2A is a cross-sectional view of a first substrate and layersprovided thereon of the optical disk according to the first example ofthe present invention;

FIG. 2B is a cross-sectional view of a second substrate and layersprovided thereon of the optical disk according to the first example ofthe present invention;

FIG. 3 shows a method for measuring the warp angle of a substrate;

FIG. 4A is a cross-sectional view of a first substrate and layersprovided thereon of an optical disk according to a second example of thepresent invention;

FIG. 4B is a cross-sectional view of a second substrate and layersprovided thereon of the optical disk according to the second example ofthe present invention;

FIG. 4C is a cross-sectional view of the optical disk according to thesecond example of the present invention;

FIGS. 5A through 5F are cross-sectional views illustrating a method forproducing an optical disk according to a third example of the presentinvention;

FIGS. 6A through 6E are cross-sectional views illustrating a method forproducing an optical disk according to a fourth example of the presentinvention;

FIG. 7 is a cross-sectional view of a mold used for forming first andsecond substrate of the optical disk according to the fourth example ofthe present invention;

FIGS. 8A through 8F are cross-sectional views illustrating a method forproducing an optical disk according to a fifth example of the presentinvention of the present invention; and

FIG. 9 is a cross-sectional view of the optical disk according to thefifth example of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described by way of illustrative exampleswith reference to the accompanying drawings.

First, the principle of the present invention will be described.

A general rewritable recording medium includes a transparent substrateformed of, for example, a polycarbonate, a plurality of dielectriclayers provided on the substrate, a recording layer interposed betweenthe plurality of dielectric layers, and optionally a reflective layer.The dielectric layers are provided for protecting the recording layeragainst contamination by water and oxygen and also for protecting thesubstrate against the high temperature of the recording layer caused atthe time of signal recording. Most rewritable recording mediums includea dielectric layer as an indispensable element.

Exemplary materials usable for the dielectric layer include oxides ofmetals or semi-metal materials, nitrides, chalcogenides, fluorides,carbides, and mixtures thereof. More specifically, exemplary materialsusable for the dielectric layer include SiO₂, SiO, Al₂O₃, GeO₂, In₂O₃,Ta₂O₅, TeO₂, TiO₂, MoO₃, WO₃, ZrO₂, Si₃N₄, Ge₃N₄, AlN, BN, TiN, ZnS,CdS, CdSe, ZnSe, ZnTe, AgF, PbF₂, MnF₂, NiF₂, SiC, or mixtures thereof,a diamond thin layer and diamond-like carbon.

Exemplary materials usable for the recording layer for an optical diskfor phase-change recording include alloys such as GeSbTe, InSbTe,InSbTeAg, GaSb, InGaSb, GeSnTe, AgSbTe. Recording mediums for recordinginformation by other mechanisms are also usable.

The dielectric layer and the recording layer are formed by, for example,sputtering or vacuum vapor deposition. When a thin layer including adielectric layer is formed on a thin substrate having a thickness of,for example, 0.6 mm, by sputtering or vacuum vapor deposition, thesubstrate is significantly warped. As described above, when such asignificantly warped substrate and another substrate which includes nothin layer and thus is not warped are assembled to produce an opticaldisk for one-side recording and reproduction, the optical disk is alsowarped. Such a warped optical disk is not usable.

EXAMPLE 1

FIGS. 1A through 1F are cross-sectional views illustrating a method forproducing an optical information recording medium (referred to simply asan optical disk) 100 according to a first example of the presentinvention. FIG. 2A is a cross-sectional view of a first substrate 1 andlayers provided thereon. FIG. 2B is a cross-sectional view of a secondsubstrate 2 and layers provided thereon.

The optical disk 100 in the first example includes the first substrate 1and the second substrate 2, each having a thickness of 0.6 mm, which areassembled together. The first substrate 1 is provided with a laminatedinformation rewriting layer 3 (FIG. 1C) including at least a dielectriclayer and a recording layer. By providing the laminated informationrewriting layer 3, the first substrate 1 warps. Accordingly, the secondsubstrate 2 is also provided with a dielectric layer 4 so as to bewarped, and the first substrate 1 and the second substrate 2 areassembled with the laminated information rewriting layer 3 and thedielectric layer 4 being inside. Thus, the optical disk 100 which isflat is obtained.

The first substrate 1 shown in FIG. 1A and the second substrate 2 shownin FIG. 1B are formed in the same step by an injection method. The firstsubstrate 1 and the second substrate 2 are formed of the same materialand have the same size and shape. For example, the first substrate 1 andthe second substrate 2 are formed of a polycarbonate and each have athickness of about 0.6 mm, a diameter of about 120 mm, and a centralhole diameter of about 15 mm. The central hole of the first substrate 1is represented by reference numeral 1 a, and the central hole of thesecond substrate 2 is represented by reference numeral 2 a. The firstsubstrate 1 and the second substrate 2 each have a guide groove (notshown) for signal recording in the top surfaces thereof (top surfaces inFIGS. 1A and 1B).

As shown in FIG. 1C, the laminated information rewriting layer 3 isprovided on the top surface of the first substrate 1. In more detail, asshown in FIG. 2A, the laminated information rewriting layer 3 includes adielectric layer 102 formed of ZnS—SiO₂ (mixture of ZnS and SiO₂), arecording layer 103 formed of GeSbTe alloy, a dielectric layer 104formed of ZnS—SiO₂, and a metal layer 105 formed of Al as a reflectivelayer, which are laminated on the top surface of the first substrate 1in this order.

The dielectric layer 102 is formed of ZnS—SiO₂ on the top surface of thefirst substrate 1 to a thickness of about 110 nm by sputtering. Next,the recording layer 103 formed of GeSbTe alloy on the dielectric layer102 to a thickness of about 30 nm by sputtering. GeSbTe alloy reversiblychanges between an amorphous state and a crystalline state in accordancewith laser light radiation. Then, the dielectric layer 104 is formed ofZnS—SiO₂ on the recording layer 103 to a thickness of about 20 nm bysputtering. The metal layer 105 is formed of Al on the dielectric layer104 to a thickness of about 100 nm by sputtering. Thus, the laminatedinformation rewriting layer 3 is formed.

As shown in FIG. 1C, the first substrate 1 is warped, by a compressivestress of the laminated information rewriting layer 3, at about 1.5° atan outer periphery thereof with the top surface on which the laminatedinformation rewriting layer 3 is projecting. The compression stress warpis mostly generated by the dielectric layers 102 and 104 formed ofZnS—SiO₂.

The warping angle is determined as shown in FIG. 3. A plate 11corresponding to the first and second substrates 1 and 2 is held by asupport 12 so as to be horizontal at the center thereof or in thevicinity thereof. Laser light 13 as collimated light having a diameterof about 1 mm is incident on the plate 11 from below. The angle of thelaser light 13 and light 14 reflected by the plate 11 is set as warpangle α(°).

The tolerable warp angle for an optical disk varies in accordance withthe apparatus for reproducing the information stored in the opticaldisk. For example, the warp angle needs to be about 0.7° or less.

When the first substrate 1 which is warped by the provision of thelaminated information rewriting layer 3 and the second substrate 2,which is flat, are assembled with an adhesive, the resultant opticaldisk has a warp angle of about 1° or more, whether the adhesive is a hotmelt resin or a UV curable resin, and is not usable.

As shown in FIG. 2B, a dielectric layer 4 is formed of Zns—SiO₂ on a topsurface of the second substrate 2 to a thickness of about 130 nm bysputtering. Thus, the second substrate 2 has the same warp angle withthat of the first substrate 1 as shown in FIG. 1D.

As shown in FIG. 1E, a UV curable resin 7 is dropped on the laminatedinformation rewriting layer 3 in the vicinity of the inner circumferenceof the first substrate 1 in a concentric manner, and the first andsecond substrates 1 and 2 are assembled together in the state of planarsymmetry with respect to each other with the layers thereon beinginside. Next, as shown in FIG. 1F, the assembled first and secondsubstrates 1 and 2 are held between glass plates 8 and 9, therebysubstantially eliminating the warp of the first and second substrates 1and 2. Then, the assembled body is irradiated with UV light 10 from theside of the second substrate 2, thereby curing the UV resin 7. Afterremoving the glass plates 8 and 9, the optical disk 100, which is flat,is obtained.

The ZnS—SiO₂ used for the dielectric layer 4 is substantiallytransparent with respect to UV light.

The tilt of the optical disk 100 produced in this manner is about 0.5°or less and thus is sufficiently usable. In the above-described example,the first and second substrates 1 and 2 are assembled using UV light.The same effect is achieved by a hot melt method using a hot melt resinor an adhesive tape. According to an alternative method, a UV curableresin which is cured slowly is applied to surfaces of the top layers onthe first and second substrates 1 and 2 by spin coating or printing.Hereinafter, such a method will be referred to as a “slowly-effective UVradiation method”. Next, the resin is irradiated by UV light to providethe resin with tackiness. Then, the first and second substrates 1 and 2are pressed together, and then the resin is completely cured. The sameeffect is achieved by this method. Alternatively, the glass plates 8 and9 for interposing the first and second substrates 1 and 2 can bereplaced with plates of other materials. The warp of the first andsecond substrates 1 and 2 can be eliminated by other methods.

In the above-described example, the first and second substrates 1 and 2are identical substrates formed by an injection method. The twosubstrates can be produced by different molds. The present invention isnot limited to the method of production of the substrates.

The dielectric layers 102 and 104 can be formed of the same material asthe dielectric layer 4. In such a case, the dielectric material which isisolated from the single target in the same sputtering apparatus can bedeposited on the first and second substrates 1 and 2. Thus, thedielectric layers 102 and 4 can be formed in the same step.

It is found that, as the thickness of the dielectric layer 4 provided onthe second substrate 2 increases, the warp angle of the second substrate2 increases. When the thickness of the dielectric layer 4 is about 130nm, the warp angle of the second substrate 2 becomes almost the same asthat of the first substrate 1. When the first and second substrates 1and 2 have substantially the same warp angle, the stresses in the twosubstrates are balanced and thus the warp of the optical disk 100 isreduced. The thickness of the dielectric layer 4 provided on the secondsubstrate 2 is preferably determined to warp the second substrate 2 toabout the same degree as the first substrate 1. Specifically, in thecase where the same material is used for the dielectric layers on thefirst substrate 1 and the second substrate 2, it is most preferable thatthe total thickness of one or more dielectric layers on the firstsubstrate 1 is equal to the total thickness of one or more dielectriclayers on the second substrate 2.

A metal layer as a reflective layer formed of, for example, Al or Au canbe provided on the dielectric layer 4 on the second substrate 2. In thiscase, UV light cannot be used for assembling the first and secondsubstrates 1 and 2. A hot melt method, a slowly-effective UV radiationmethod or the like can be used. Providing the metal layer on thedielectric layer 4 is advantageous in improving the external appearancesince, even when bubbles are present on the surface on which the resinis applied, such bubbles are covered by the metal layer.

The laminated information rewriting layer 3 can have a differentstructure from described above. The metal layer 105 can be eliminated,or each or both dielectric layers include a plurality of layers formedof different materials.

EXAMPLE 2

FIGS. 4A through 4C show an optical information recording medium(referred to simply as an optical disk) 200 according to the secondexample of the present invention. FIG. 4A is a cross-sectional view of afirst substrate 5 and layers provided thereon, and FIG. 4B is across-sectional view of a second substrate 6 and layers providedthereon. FIG. 4C is a cross-sectional view of the optical disk 200including the first and second substrates 5 and 6 and the layersprovided thereon.

As shown in FIG. 4A, provided sequentially on the first substrate 5 area dielectric layer 202 formed of ZnS—SiO₂, a dielectric layer 203 formedof GeN, a recording layer 204 formed of GeSbTe alloy, a dielectric layer205 formed of GeN, a metal layer 206 as a reflective layer formed of Al,and an overcoat layer 207.

As shown in FIG. 4B, provided sequentially on the second substrate 6 area dielectric layer 212 formed of ZnS—SiO₂, a dielectric layer 213 formedof GeN, a dielectric layer 215 formed of GeN, a metal layer 216 as areflective layer formed of Al, and an overcoat layer 217.

As shown in FIG. 4C, the first and second substrates 5 and 6 areassembled together with the overcoat layers 207 and 217 as innermostlayers and with a resin layer 221 interposed between the overcoat layers207 and 217. Hardcoat layers 201 and 211 are provided on outer surfacesof the assembled first and second substrates 5 and 6.

The layers provided on the second substrate 6 are the same as the layersprovided on the first substrate 5 except that the recording layer 204 isprovided above the first substrate 5. The dielectric layers, the metallayer and the overcoat layer are provided on both the first and secondsubstrates 5 and 6. Each of the dielectric layers, the metal layers andthe overcoat layers on the first and second substrates 5 and 6respectively have the same thicknesses.

In such a structure, substantially the same stress acts on the first andsecond substrates 5 and 6, and as a result, the first and secondsubstrates 5 and 6 are warped to substantially the same degree. Evenwhen the stress acting on the first and second substrates 5 and 6changes in accordance with the passage of time, the change of the stressacting on the first substrate 5 and the change of the stress acting onthe second substrate 6 are substantially the same. Accordingly, theoptical disk 200 is kept flat for a long period of time.

When the optical disk 200 is seen from the side of the second substrate6, light which is incident on the second substrate 6 and reflected bythe metal layer 216 is colored yellow by the dielectric layers 213 and215 formed of GeN. consequently, the unrecordable surface of the opticaldisk 200 looks yellow.

When the optical disk 200 is seen from the side of the first substrate5, light which is incident on the first substrate 5 and reflected by themetal layer 206 is colored yellow by the dielectric layers 203 and 205formed of GeN and also colored blue by the recording layer 204 formed ofGeSbTe alloy. Since the coloring degree of blue is stronger than thecoloring degree of yellow, the recordable surface of the optical disk200 looks blue.

Accordingly, the recordable surface and the unrecordable surface areeasily distinguishable.

EXAMPLE 3

FIGS. 5A through 5F are cross-sectional views illustrating a method forproducing an optical information recording medium (referred to simply asan optical disk) 300 according to a third example of the presentinvention.

The optical disk 300 in the third example includes a first substrate 21and a second substrate 22, each having a thickness of 0.6 mm, which areassembled together. The first substrate 21 is provided with, on the topsurface thereof, a laminated information rewriting layer 23 (FIG. 5C)including at least a dielectric layer and a recording layer. Byproviding the laminated information rewriting layer 23, the firstsubstrate 21 warps. Accordingly, the second substrate 22 is providedwith a resin layer 25 on the bottom surface thereof so that the resinlayer 25 is not opposed to the laminated information rewriting layer 23when the first and second substrates 21 and 22 are assembled. The resinlayer 25 is formed of a resin which shrinks when being cured. Byproviding the resin layer 25, the second substrate 22 is also warped.Then, the first and second substrate 21 and 22 are assembled together.Thus, the optical disk 300, which is flat, is obtained.

The first substrate 21 shown in FIG. 5A and the second substrate 22shown in FIG. 5B are formed in the same step by an injection method. Thefirst substrate 21 and the second substrate 22 are formed of the samematerial and have the same size and shape. For example, the firstsubstrate 21 and the second substrate 22 are formed of a polycarbonateand each have a thickness of about 0.6 mm, a diameter of about 120 mm,and a central hole diameter of about 15 mm. The central hole of thefirst substrate 21 is represented by reference numeral 21 a, and thecentral hole of the second substrate 22 is represented by referencenumeral 22 a. The first substrate 21 and the second substrate 22 eachhave a guide groove (not shown) for signal recording in the top surfacesthereof (top surfaces in FIGS. 5A and 5B).

As shown in FIG. 5C, the laminated information rewriting layer 23 isprovided on the top surface of the first substrate 21. The laminatedinformation rewriting layer 23 has the same structure as that of thelaminated information rewriting layer 3 in the first example. The firstsubstrate 21 is warped at about 1.5° at an outer periphery thereof withthe top surface on which the laminated information rewriting layer 23 isprojecting.

As shown in FIG. 5D, the resin layer 25 formed of a resin which has itsvolume reduced when being cured is formed on the bottom surface of thesecond substrate 22, i.e., the surface which is not opposed to the firstsubstrate 21 when the first and second substrates 21 and 22 areassembled. Specifically, a UV curable resin is dropped on the bottomsurface of the second substrate 22, and the second substrate 22 is spun(spin coating), thereby forming the resin layer 25 having a uniformthickness of about 5 μm. Then, the resin layer 25 is irradiated with UVlight. The UV curable resin shrinks when being cured, and thus thevolume thereof changes by about 10% or more. As a result, tensile stressacts on the second substrate 22. Thus, as shown in FIG. 5E, the secondsubstrate 22 is warped, with the surface on which the resin layer 25 isnot provided projecting. In other words, the surface of the secondsubstrate 22 in which the guide groove is formed is projecting.

Then, as shown in FIG. 5E, a UV curable resin 27 is dropped on thelaminated information rewriting layer 23 in the vicinity of the innercircumference of the fist substrate 21 in a concentric manner, and thefirst and second substrates 21 and 22 are pressure-contacted to eachother in the state of planar symmetry with respect to each other withthe laminated information rewriting layer 23 being inside and the resinlayer 25 outside. Next, as shown in FIG. 5F, the assembled first andsecond substrates 21 and 22 are held between glass plates 28 and 29,thereby substantially eliminating the warp of the first and secondsubstrates 21 and 22. Then, the assembled body is irradiated with UVlight 30 from the side of the second substrate 22, thereby curing the UVresin 27. After removing the glass plates 28 and 29, the optical disk300 is obtained.

The resin layer 25 is formed of a material which is substantiallytransparent with respect to UV light so as to allow for theabove-described assembly.

The tilt of the optical disk 300 produced in this manner is about 0.5°or less and thus is sufficiently usable. In the above-described example,the first and second substrates 21 and 22 are assembled using UV light.The same effect is achieved by a hot melt method, a method using anadhesive tape or a slowly-effective UV radiation method.

The resin layer 25 can be formed of a material which is not transparentwith respect to visible light. In such a case, the assembly of the firstand second substrates 21 and 22 cannot be performed by UV radiation, butcan be performed by a hot melt method, a slowly-effective UV radiationmethod or the like. Use of non-transparent resin for the resin layer 25is advantageous in improving the external appearance since, even whenbubbles are present on the surface on which the resin is applied, suchbubbles are covered by the non-transparent resin layer 25.

In the above-described example, the resin layer 25 is formed by spincoating. The resin layer 25 can be formed by, for example, printing, inwhich case, a graphics can be provided on the resin layer 25. In thismanner, the external appearance of the optical disk is further improved,an area in which the user records a list of stored information can beprovided.

EXAMPLE 4

FIGS. 6A through 6E are cross-sectional views illustrating a method forproducing an optical information recording medium (referred to simply asan optical disk) 400 according to a fourth example of the presentinvention. The optical disk 400 includes a first substrate 31 and asecond substrate 32 assembled together. FIG. 7 is a cross-sectional viewof a mold 60 used for forming the first and second substrate 31 and 32.

The first substrate 31 and the second substrate 32 each have a thicknessof 0.6 mm. The first substrate 31 is provided with a laminatedinformation rewriting layer 33 (FIG. 6C) including at least a dielectriclayer and a recording layer. By providing the laminated informationrewriting layer 33, the first substrate 31 warps. Accordingly, thesecond substrate 32 is also warped, so that the optical disk 400, whichis flat, is obtained by assembling the first substrate 31 and the secondsubstrate 32.

The first substrate 31 shown in FIG. 6A and the second substrate 32shown in FIG. 6B are formed in the same step by an injection method. Thefirst substrate 31 and the second substrate 32 are formed of the samematerial and have the same size and shape. For example, the firstsubstrate 31 and the second substrate 32 are formed of a polycarbonateand each have a thickness of about 0.6 mm, a diameter of about 120 mm,and a central hole diameter of about 15 mm. The central hole of thefirst substrate 31 is represented by reference numeral 31 a, and thecentral hole of the second substrate 32 is represented by referencenumeral 32 a. The first substrate 31 and the second substrate 32 eachhave a guide groove (not shown) for signal recording in the top surfacesthereof (top surfaces in FIGS. 6A and 6B).

The formation of the first and second substrates 31 and 32 will bedescribed with reference to FIG. 7.

The mold 60 shown in FIG. 7 includes upper and lower molds 61 and 62opposed to each other. A space 63 is formed between the upper and lowermolds 61 and 62. A resin (for example, a polycarbonate) in a meltedstate is injected into the space 63, thereby forming each of the firstand second substrates 31 and 32 (FIGS. 6A through 6E). The lower mold 62includes a stamper 64 for forming a guide groove for signal recording inthe surface of each of the first and second substrates 31 and 32. Themold 60 is maintained at a high temperature of, for example, about 100°C. or more, so that the injected resin is not cooled rapidly and thegroove pattern of the stamper 64 is accurately transferred to thesurface of each of the first and second substrates 31 and 32.

As shown in FIG. 6C, the laminated information rewriting layer 33 isprovided on a top surface of the first substrate 31. The laminatedinformation rewriting layer 33 has the same structure as that of thelaminated information rewriting layer 3 in the first example. The firstsubstrate 31 is warped at about 1.5° at an outer periphery thereof withthe top surface on which the laminated information rewriting layer 33 isprojecting.

The second substrate 32 is formed so that the surface in which the guidegroove is formed projects. The warp angle of the second substrate 32 isadjusted by changing the injection conditions.

For example, when the upper and lower molds 61 and 62 (FIG. 7) are setto have the temperature in condition 1 or 2 shown in Table 1, the secondsubstrate 32 is warped to substantially the same degree as that of thefirst substrate 31. The warp angle is larger by condition 2 than bycondition 1.

TABLE 1 Condition 1 Condition 2 Temperature Mold 61 125° C. 128° C. Mold62 130° C. 130° C.

Conditions 1 and 2 are mere examples for a specific mold. The conditionsfor warping the first and second substrates 31 and 32 change inaccordance with, for example, the structure of the mold and the materialof the substrate. Since the birefringence characteristics of thesubstrates changes in accordance with the temperature of the mold, thetemperature of the mold needs to be set in consideration of variouscharacteristics of the substrates.

As shown in FIG. 6D, a UV curable resin 37 is dropped on the laminatedinformation rewriting layer 33 in the vicinity of the innercircumference of the first substrate 31 in a concentric manner, and thefirst and second substrates 31 and 32 are assembled together in thestate of planar symmetry with respect to each other with the laminatedinformation rewriting layer 33 being inside. Next, as shown in FIG. 6E,the assembled first and second substrates 31 and 32 are held betweenglass plates 38 and 39, thereby substantially eliminating the warp ofthe first and second substrates 31 and 32. Then, the assembled body isirradiated with UV light 40 from the side of the second substrate 32,thereby curing the UV resin 37. After removing the glass plates 38 and39, the optical disk 400 is obtained.

The tilt of the optical disk 400 produced in this manner is about 0.5°or less and thus is sufficiently usable. In the above-described example,the first and second substrates 31 and 32 are assembled using UV light.The same effect is achieved by a hot melt method using a hot melt resin,a method using an adhesive or a slowly-effective UV radiation method.

In this example, the second substrate 32 is warped while being formed byan injection method. Any other method which warps the second substrate32 is usable.

EXAMPLE 5

FIGS. 8A through 8F are cross-sectional views illustrating a method forproducing an optical information recording medium (referred to simply asan optical disk) 500 according to a fifth example of the presentinvention. FIG. 9 is a detailed cross-sectional view of the optical disk500.

The optical disk 500 includes a first substrate 41 and a secondsubstrate 42, each having a thickness of 0.6 mm, which are assembledtogether. The first substrate 41 is provided with a laminatedinformation rewriting layer 43 (FIG. 8C) including at least a dielectriclayer and a recording layer. By providing the laminated informationrewriting layer 43, the first substrate 41 warps. Accordingly, a resinlayer 44 formed of a resin which shrinks when being cured is furtherprovided on the laminated information rewriting layer 43, therebysubstantially eliminating the warp of the first substrate 41. Then, thefirst substrate 41 and the second substrate 42 are assembled with theresin layer 44 being inside. Thus, the optical disk 500, which is flat,is obtained.

The first substrate 41 shown in FIG. 8A and the second substrate 42shown in FIG. 8B are formed in the same step by an injection method. Thefirst substrate 41 and the second substrate 42 are formed of the samematerial and have the same size and shape. For example, the firstsubstrate 41 and the second substrate 42 are formed of a polycarbonateand each have a thickness of about 0.6 mm, a diameter of about 120 mm,and a central hole diameter of about 15 mm. The central hole of thefirst substrate 41 is represented by reference numeral 41 a, and thecentral hole of the second substrate 42 is represented by referencenumeral 42 a. The first substrate 41 and the second substrate 42 eachhave a guide groove (not shown) for signal recording in the top surfacesthereof (top surfaces in FIGS. 8A and 8B).

As shown in FIG. 8C, the laminated information rewriting layer 43 isprovided on the top surface of the first substrate 41. The laminatedinformation rewriting layer 43 has the same structure as that of thelaminated information rewriting layer 3 in the first example. As shownin FIG. 8C, the first substrate 41 is warped at about 1.5° at an outerperiphery thereof with the top surface on which the laminatedinformation rewriting layer 43 is projecting.

As shown in FIG. 8D, the resin layer 44 formed of a resin which shrinkswhen being cured is formed on the laminated information rewriting layer43. The warp of the first substrate 41 is substantially eliminated bythe stress accompanying the shrinkage of the resin layer 44.Specifically, a UV curable resin is dropped on the bottom surface of thelaminated information rewriting layer 43, and the first substrate 41 isspun (spin coating), thereby forming the resin layer 44 having a uniformthickness of about 5 μm. Then, the resin layer 44 is irradiated with UVlight. The UV curable resin shrinks when being cured, and thus thevolume thereof changes by about 10% or more. As a result, tensile stressacts on the first substrate 44. Thus, as shown in FIG. 8E, the firstsubstrate 41 is deformed so as to reduce the warp thereof.

Then, a UV curable resin 47 is dropped on the laminated informationrewriting layer 43 in the vicinity of the inner circumference of thefirst substrate 41 in a concentric manner, and the first and secondsubstrates 41 and 42 are assembled together in the state of planarsymmetry with respect to each other with the layers 43 and 44 beinginside. Next, as shown in FIG. 8F, the assembled first and secondsubstrates 41 and 42 are held between glass plates 48 and 49. Then, theassembled body is irradiated with UV light 50 from the side of thesecond substrate 42, thereby curing the UV resin 47. After removing theglass plates 48 and 49, the optical disk 500 is obtained.

As shown in FIG. 9, the laminated information rewriting layer 43provided on the first substrate 41 includes a dielectric layer 122, arecording layer 123, a dielectric layer 124 and a metal layer 125. Anover-coat layer 126 is provided on the laminated information rewritinglayer 43. The resin layer 44 is provided on the second substrate 42. Thefirst and second substrates 41 and 42 are assembled together with thelayers 126 and 44 being inside. Hardcoat layers 121 and 127 are providedon outer surfaces of the assembled first and second substrates 41 and42.

The tilt of the optical disk 500 produced in this manner is about 0.5°or less and thus is sufficiently usable. In the above-described example,the first and second substrates 41 and 42 are assembled using UV light.The same effect is achieved by a hot melt method using a hot melt resin,an adhesive tape or a slowly-effective UV radiation method.

In the case where a resin layer is provided on the surface of the secondsubstrate 42 in which the guide groove is formed, the resin layerpreferably has a smaller tensile stress than that of the resin layer 44.Thus, the tilt of the optical disk is reduced. Specifically, a thinnerlayer of a resin which is the same as that of the resin layer 44 isused, or a resin having a smaller shrinkage ratio when being cured thanthat of the resin layer 44 is used.

In the case where the resin layer is provided on the second substrate42, the adhesiveness between the first and second substrate 41 and 42 isimproved. Without the resin layer on the second substrate 42, theadhesiveness between the adhesive for bonding the first and secondsubstrate 41 and 42 and the material of the second substrate 42 can beinsufficient. When the resin layer 44 formed on the laminatedinformation rewriting layer 43 is substantially transparent with respectto UV light, the first and second substrate 41 and 42 can be assembledusing UV light as shown in FIG. 8F.

The resin layer 44 can be formed of a material which is not transparentwith respect to visible light. In such a case, the assembly of the firstand second substrates 41 and 42 cannot be performed by UV radiation, butcan be performed by a hot melt method, a slowly-effective UV radiationmethod or the like. Use of non-transparent resin for the resin layer 44is advantageous in improving the external appearance since, even whenbubbles are present on the surface on which the resin is applied, suchbubbles are covered by the non-transparent resin layer 44.

In the above-described example, the resin layer 44 is formed by spincoating. The resin layer 44 can be formed by, for example, printing, inwhich case, a graphics can be provided on the resin layer 44. In thismanner, the external appearance of the optical disk is further improved.

In the first through fifth examples, each of the f irst and secondsubstrates has a thickness of about 0.6 mm. The present invention isapplicable to substrates having other thicknesses.

In the case where the first and second substrates each have a thicknessof more than 0.8 mm, simply assembling the two substrates in the statewhere the first substrate is warped by providing a laminated informationrewriting layer reduces the warp angle of the optical disk, for example,to about 0.7° or less. Thus, the present invention is especiallyeffective to an optical disk produced by assembling substrates eachhaving a thickness of less than 0.8 mm.

In the first through fifth examples, the second substrate has a guidegroove for signal recording as well as the first substrate. Since therecording layer is not provided on the second substrate, the guidegroove in the second substrate can be eliminated.

As described above, an optical information recording medium for one-siderecording and reproduction including two thin substrates assembledtogether according to the present invention is flat and has asufficiently small tilt. The optical information recording mediumaccording to the present invention promotes development of and providesoptical disk systems capable of higher density recording withoutrequiring a reduction wavelength of the laser light or an increase inthe numerical aperture of the objective lens.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. An optical information recording medium,comprising: a first substrate; a plurality of first dielectric layers, arecording layer for signal recording provided between the plurality ofdielectric layers, and a metal layer, which are provided on a surface ofthe first substrate; a second substrate; and a thin layer, including atleast one second dielectric layer and a metal layer, on which a signalis unrecordable, the thin layer being provided on a surface of thesecond substrate, wherein the first substrate and the second substrateare assembled together with the plurality of first dielectric layers,the at least one second dielectric layer, the recording layer, the metallayer, and the thin metal layer being interposed therebetween.
 2. Anoptical information recording medium according to claim 1, wherein theplurality of first dielectric layers and the at least one seconddielectric layer has a substantially equal total thickness.
 3. Anoptical information recording medium, comprising: a first substrate; atleast a dielectric layer and a recording layer for signal recordingprovided on a surface of the first substrate; a second substrate; and aresin layer formed of a resin which shrinks when being cured, the resinlayer being provided on a surface of the second substrate, wherein thefirst substrate and the second substrate are assembled together with thedielectric layer and the recording layer being interposed therebetween,and the resin layer is not interposed therebetween.
 4. An opticalinformation recording medium according to claim 3, wherein the resinlayer is formed of a UV curable resin.